Devices that provide both input and output to the system unit are called
input/output (I/O) devices. A modem is an I/O device as is a
communications local area network (LAN) or a wide area network (WAN)
A modem is an electronic connection, usually via telephone, between your
computer and others. Modems exchange files and data with other computers
via local networks or telephone connections. Fax moderns link a fax machine
to a modem for the direct transfer of an image to the originator. Modems
can be stand alone or board units. Stand alone units may be acoustic modems
in which a cradle holds a telephone headset and you dial the phone number of
the computer system to establish a link or direct-connect modems that plug
directly into a phone line. Board modems are built onto plug-in boards that
insert into the expansion ports inside the system unit of the computer.
Figure 3-13 shows three different types of modems.
Figure 3-13. Modems.